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Re: Surface vs. Abstract Syntax, was: RE: What do the ontologists want

From: pat hayes <phayes@ai.uwf.edu>
Date: Fri, 18 May 2001 16:58:20 -0500
Message-Id: <v04210133b72b45082fc8@[205.160.76.183]>
To: "Jonathan Borden" <jborden@mediaone.net>
Cc: www-rdf-logic@w3.org
>Peter Crowther wrote:
>
> >
> > > From: Sergey Melnik [mailto:melnik@db.stanford.edu]
> > [...]
> > > can anyone criticizing reification suggest a more
> > > suitable mechanism
> > > for handling the aforementioned features that makes both
> > > programmers and logicians happy?
> >
> > Ditch RDF and layer a logic directly on XML?  Just a thought...
> > the problem
> > is that it loses a lot of the work currently being put into the
> > Semantic Web
> > and being described using RDF, unless there's a well-defined
> > migration path.
> > But it would give much more flexible structures and a far simpler way of
> > denoting what has formally defined semantics versus what is simply a data
> > structure.
>
>Alot is being said in this thread (this message is just a convenient one to
>respond to) about dissatisfaction with the RDF syntax -- and then there are
>responses and responses to responses -- so I want to remind people about the
>difference between the _surface syntax_ as pat hayes describes what is
>termed the "RDF (XML) syntax", and the _abstract syntax_ what is termed the
>"RDF Model" in RDF 1.0 M & S.

Just for the record, my concerns with the adequacy of triples and so 
on apply to the RDF model.

>There have been several suggestions to modify the surface RDF syntax,
>notably N3, as well Sergey and I have proposed simplified RDF syntaxes e.g.
>http://www.openhealth.org/RDF/rdf_Syntax_and_Names.htm with an
>implementation http://www.openhealth.org/RDF/rdfExtractify.xsl which has the
>property of interpreting _arbitrary_ XML as RDF triples.
>
>What then is the relationship between the abstract syntax of XML and the
>abstract syntax of RDF. This sort of topic has been long discussed in the
>markup world as
>"Groves" (for intro see: http://www.prescod.net/groves/shorttut/ , for full
>XML grove see: http://www.openhealth.org/XSet).
>
>Essentially the 'full' grove of a document describes every niggley detail
>including whitespace, whether attributes are quoted using single or double
>quotes and what the order of attributes is. These details are deemed
>unimportant to XML applications. The XML Infoset
>http://www.w3.org/TR/xml-infoset/ serves to specify a common _subset_ of the
>full XML abstract syntax that is useful to most XML software.
>
>The XML Infoset (read XML abstract syntax) represents order of child
>elements (but not order of attributes) whether an information item arises
>from an attribute or elemenet etc.
>
>The RDF abstract syntax can be viewed as a further bare subset of the XML
>infoset where element order is not represented. This allows an RDF abstract
>syntax to be represented by a single relational table (p,s,o) where an XML
>infoset is represented by a more complex structure e.g. a DOM. So the
>advantage of RDF is the simplicity of storage, yet to represent element
>order (e.g. a container) additional constructs are added which make life
>difficult (containers). In XML every element is naturally a container and
>containment is a very natural part of the abstract (and surface) syntax.
>
>How does N3 relate? N3 is simply an alternate _surface syntax_ that is
>easier for humans to write, it does not change the _abstract syntax_.

Well, yes and no. N3 seems to extend the abstract syntax in some 
nontrivial ways as well, eg it has universal quantification and 
implication. That goes way beyond the RDF model. In fact, N3 seems to 
be an example of the kind of thing we have been talking about: a 
language which differs from (extends) RDF, but which is 
*implementable* in RDF triplets. (The same could be said of 
DAML+OIL.) However, I confess to not really knowing what the abstract 
syntax of N3 is supposed to be.

> >
> > The alternative appears to be to accept that RDF will be used as a very,
> > very verbose encoding of LISP cons cells; and that some part of those
> > structures might be used to represent something formal, but that a large
> > part will straight data structure, or be glue that could be encoded and
> > processed more easily using a richer syntax.
> >
> > 		- Peter
> >
> > [insert back view of Peter running down infinitely long corridor
> > towards the
> > end marked "RDF Logic, Holy Grail, World Peace and Emergency Exit" pursued
> > by mixed crowd of logicians and RDF enthusiasts waving pitchforks and
> > torches]
>
>Yes this would be very very painful, using triples merely to represent
>lists. One possibility is to introduce containers in a more natural fashion,
>perhaps using a canonical triple ordering (e.g.
>http://www.openhealth.org/RDF/RDFmediatype.html). Another would be to bite
>the bullet and make containment and ordering a natural feature of the RDF
>abstract syntax. The current container mechanism _is_ painful.

But this goes beyond just adding containers. LISP lists are used to 
implement everything: expressions, in particular, are encoded as 
lists. That is why I prefer to say "S-expressions", to emphasise that 
this is a general-purpose datastructuring technique, not merely one 
kind of datatype among many. (It may be that I have misunderstood 
you, and that you mean 'containment' in a much more comprehensive 
sense than simply an additional datatype; in which case I would agree 
with you, I think.) This proposal for thinking about RDF would make 
the RDF into an implementation language in which to write expressions 
in some other language (which would have a semantics), rather than 
just adding a handy datatype to RDF, in the way that DAML adds lists 
as a shorthand for nested-triple structures of a certain kind (which 
would be too painful to spell out in detail).

Pat Hayes

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Received on Friday, 18 May 2001 17:58:20 UTC

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